Digital capture has many advantages over traditional film capture. One of them is the ability to use a single sensor to capture images in different lighting conditions, whereas in the film realm, films with different sensitivity are used in different lighting conditions.
Digital capture can be easily adapted to new lighting conditions by sampling a digitally acquired gray patch image. A chromatic adaptation transform is performed using sampled points of the captured gray patch image. This transformation complies with the adaptation to the new lighting conditions, however, often this technique is not sufficient, especially in cases where highlight and shadow reference points are required.
Current technique to overcome deficiencies caused by different lighting conditions, while acquiring a digital image, using digital capture are not completely adequate. For example, an image is acquired by a digital camera at non-standard lighting conditions. The acquired image is displayed on a color display, at this stage the user is asked to select areas on the displayed image that were gray on the captured object, for example, a gray patch or a gray area in the captured object. The red, green, and blue (RGB) values of the specified area are sampled, and are used for camera recalibration to the lighting conditions. This method is being used in some camera products, and is called mid-tone picker calibration. Although this method is satisfactory in the case of most acquired images, and results in adequate colors, it often requires additional handling to perfect the image. In certain cases, highlight and shadow areas will show deviation from gray.
FIG. 1 shows a prior art method in which an image is acquired in camera RGB 11 space. The acquired image camera RGB space are converted to reference output medium metric (romm) RGB 13 space, using a camera to romm matrix multiplication 12. Romm space is typically used for efficient image processing. The mid-tone picker calibration is better used with temperature and tint values. Those two values are computed by applying a temperature and tint transformation 14 on the romm RGB space values 13.
At this stage, a transformation is made on the entire image from the adjusted temperature and tint values by the mid-tone picker calibration into modified romm RGB 15 values. This multiplication is done in the cone space, where a Von Kries transformation is performed, according to temperature and tint values. The modified romm RGB is converted using International Color Consortium (ICC) profile transformation 16 into designated output RGB 17 space.
This mid-tone calibration method fails to preserve the ratio between the colors in all the instances, it is mostly evident when the ratio is 1:1, typically expected for gray values. The currently used highlight and shadow recalibration methods, will fail to produce good results when using in conjunction with the flow described above. The reason for that is due to the fact that the highlight and shadow recalibration will not be preserved at the output RGB 17 space. Therefore, a tool that recalibrates the gray in the highlight and shadow values would be valuable.
The enclosed invention proposes a method to overcome this deficiency caused by the mid-tone picker calibration method. A transformation for highlight and shadow reference points, when the chromatic adaptation transformation is done by a temperature and tint finding method would be desirable.